With conventional roller lifters, each lifter rolls over the cam surface to help eliminate friction, while the other end is engaged with a push rod seat firmly fixed to the roller body. Its job is merely to follow the cam profile, producing action to open and close the valves. This solid lifter action is inflexable and remains at an adjusted valve lash setting. As the valve train components wear with this solid type lifter, valve lash settings must be periodically reset.
With conventional types of hydraulic lifters, each valve actuating push rod seats in a plunger axially slideable in a lifter body. The lifter body rides on one lobe of the cam as it slides across the cam's surface creating friction. Oil from the engine lubrication system is introduced under pressure between the body and plunger. At increased engine speeds, the valve lifter assembly expands axially to tighten the linkage in the valve actuating train, thus maintaining a zero valve lash adjustment according to engine wear and heat expansion. Many lifters are designed to trap oil in the lifters pressure chamber during high speeds while the valves are near the floating point. Lifter pump up prevents the valve from returning back to its seat and creates engine misfire.
Hydraulic rollers are used widespread in diesel engines, their construction being a single body and a single plunger within the body and consist of a roller on the bottom end.